A typical vehicle features a driving engine, a power transmission system, and a final drive with two output differential axles connected to one or more sets of driven wheels. A conventional power transmission system includes a single input shaft and a single output shaft, and usually operates with stepwise speed ratio changes defined by the mechanical gear ratios contained therein. The power from the engine is delivered through the input shaft to the power transmission system, and then through the output shaft to the final drive. The final drive distributes the driving power through each differential axle to each driven wheel on an equal-torque basis. To achieve a desired vehicle speed, the engine speed is varied between each speed ratio change in the power transmission system. The wide variation in driving engine speed reduces fuel efficiency and increases exhaust emissions.
When road surface variations at each wheel produce different coefficients of friction, the lower wheel driving torque of the two wheels limits the effective driving torque, which is twice the lowest wheel torque. The application of torque in excess of the lowest wheel torque level results in spinning of the vehicle wheel.
Infinitely variable power transmission (IVT) systems and continuously variable power transmission (CVT) systems have been developed to provide continuous speed ratio changes, thereby improving vehicle fuel efficiency, driving comfort, and reducing vehicle exhaust emissions by permitting the vehicle engine to maintain a relatively constant speed. Both IVT and CVT systems are capable of providing continuous speed ratio change between the driving engine and driven wheels, however, the output speed of an IVT can be reduced to zero and even reversed, while the output speed of a CVT cannot. This IVT capability is an important feature, because devices utilized to assist in vehicle launch (i.e. transition from a stopped state to a moving state), such as torque converters or clutches, can be eliminated.
There are two types of IVT systems: hydro-mechanical IVT systems and electro-mechanical IVT systems. Although a wide variety of configurations are possible, the vast majority of IVT systems operate on a power-split concept. They feature a single input for receiving power from the driving engine and a single output shaft for delivering the power to the final drive and associated driven wheels. They further employ some form of power splitting devices, allowing the power at the input shaft to be converted partially or fully converted into non-mechanical forms such as hydraulic power or electric power, and then reconverted back to mechanical forms before leaving at the output shaft.
Hydro-mechanical IVT systems are known in the art to have several drawbacks. First, hydraulic drives are not suitable for high-speed operation. This, to a large degree, limits hydro-mechanical IVT systems to non-automotive applications. Secondly, hydrostatic pump or motors utilized in hydro-mechanical IVT systems are very noisy when operated at pressures greater than 5,000 PSI. Finally, hydrostatic pumps and motors are not conducive to shaft-concentric and compact transmission designs. In addition, the positions of the hydrostatic units such as pumps or motors within the IVT system may be subjected to various mechanical constraints.
The electro-mechanical IVT systems overcome several of the aforementioned problems. Recent developments in electro mechanical systems have demonstrated, among other features, advantages efficiency, controllability, and system flexibility. Furthermore, with the addition of energy storage systems, electro-mechanical IVT systems can also function as power regulators, buffering output power fluctuations, thereby providing vehicle drive system hybridization options
U.S. Pat. Nos. 5,907,191, 5,914,575, 5,991,683 and 5,920,160 each assigned to Toyota Jidosha Kabushiki Kaisha of Toyota, Japan disclose a single-node electro-mechanical power split transmission known in the industry as the Toyota Hybrid System (THS). The THS employs a single planetary gear system comprising a sun gear, a ring gear, and a planetary carrier as a power splitting device, such that the THS device is categorized as an output power split system. In the THS, the planetary carrier is connected to the input shaft to receive power from the driving engine. The associated sun gear is connected to an electric motor/generator. The ring gear is connected to a second motor or generator and to the output shaft that delivers the power to the driving wheels through a differential. The THS has an adequate speed ratio range for compact passenger car applications. Within the transmission speed range, there is a point where zero power passes through the electric motor path. Power is transmitted mechanically from the input shaft to the output shaft. This point is defined as the node point at which the transmission yields the maximum efficiency.
While the THS is suitable for providing infinitely variable speed and some level of vehicle hybridization, the output power to each driving wheel cannot be individually controlled. When driving in uneven terrain having varied surface coefficients of friction, it is highly desirable to match the driving power supplied to each individual driven wheel to different driving requirements. Driving the driven wheels at different speeds and individually controlling the driving torque when traveling on a slippery surface or around a curve has the distinct advantages of avoiding vehicle deformation, reducing tire wear, attaining improved traction, and enhancing vehicle dynamic stability.
U.S. Pat. No. 5,947,855 assigned to Deere & Company of Moline, Ill. discloses a vehicle hybrid wheel drive system. The hybrid drive system includes a pair of summing gears, each having an output shaft coupled to a respective driven wheel. Each summing gear also features a first input coupled to a drive shaft and a second input coupled to a respective electric motor. The drive shafts in both summing gears are operatively connected to a common shaft that in turn connects to driving internal combustion engine and an electric generator. This system has a single node point where zero power passes through the electric motor path, and since the torque splitting takes place at the power input, this system is classified as an input power split system. An input power split system is most suited for high-speed operation beyond the node point because at slow vehicle speed operation, below the node point, excessive internal power circulation takes place within the power transmission system. The power that passes through the electric path can become several times greater than the mechanical transmission power. This significantly reduces the efficiency, thereby offsetting the benefit otherwise produced by the IVT system.
In addition, the mechanical system disclosed in U.S. Pat. No. 5,947,855 is complex, having two sets of identical Ravigneaux compound planetary trains. This not only affects the compactness of the drive system, limiting the application scope, but also increases the cost.
Accordingly, it would be highly desirable to provide a simple power split hybrid wheel drive system which is capable of providing an infinitely variable speed ratio and of controlling both torque and power application to multiple driven wheels of a vehicle, so as to provide for improved vehicle fuel efficiency, reduced driving engine emissions, and enhanced vehicle dynamic stability.